The majority of lipo accidents have occurred while the battery was being charged or discharged with a charger or after a crash. Of the batteries that have burst into flames on a charger, most have contained a defect induced by a crash, mis-handling, old age, or a manufacturing defect.
There is also the fact that many “budget” after market chargers employ highly suspect component quality and defective operational code, which can and has caused battery failures.
It’s pretty darn hard to take immediate action to contain or extinguish a fire when the owner is not present to act.